Telephone signals are provided to subscribers (customers) through the public switched telephone network. Necessarily, any terminal equipment connected to this network by or for a subscriber must meet certain specifications in order to function properly.
To connect subscriber equipment such as data modems, facsimile machines, (non-cellular) portable telephones, speaker telephones, message answering machines, etc. to the analog public switched telephone network, one must provide an interface or data access arrangement to bridge any incompatibilities between the network and the subscriber equipment. In addition to complying with network protocols, since subscriber equipment are typically four-wire devices with separate transmit and receive pairs, the interface must separate the analog signals on the network into discrete transmit and receive signals (and vice versa). Finally, the interface must electrically isolate the telephone network from the subscriber equipment.
Isolation line transformers have historically been used in DAA circuits to provide the required separation between the network and the subscriber. However, such transformers are physically large, heavy and costly and, thus, are not well-suited for applications requiring the interface to have minimal volume and weight (e.g., small portable computers and data entry devices). In such reduced volume/weight applications, a substitute such as optical couplers (optocouplers), must be used to provide the requisite function.
Optocouplers are formed of a light emitter, typically an infrared light emitting diode, and a photodetector. The emitter and photodetector are mounted on a lead frame side-by-side, or vertically facing one another on a jogged lead frame. In a hybrid DAA circuit, typically, a plurality of optocouplers are mounted on a substrate along with a variety of discrete components and integrated circuits that they connect to in the circuit.
Although optocouplers provide a DAA with a smaller profile and reduced weight, they also present certain difficulties. First, the smaller profile of the DAA circuit on the substrate may adversely affect the DAA function of providing electrical isolation between the network and the subscriber equipment. Such isolation may be difficult to accomplish when the optocouplers of the DAA circuit are physically located close to one another. Second, severe components testing, such as pressure-cooker tests, used in quality assurance evaluations may damage the components. In a typical quality assurance test, a DAA is placed in a pressure-cooker for 24 hours at approximately 120.degree. C. and 2 atmospheres and then checked to see if it remains functional. If the circuit board of the DAA becomes exposed during such tests, water may migrate to the inside of the optocouplers or other components and create an arc path, causing the DAA to fail electronically.
In part to protect against environmental conditions, circuit boards are often covered with an encapsulating material on the circuit side of the substrate. Such a circuit board, however, is not completely sealed to the outside environment, and is still susceptible to the damage discussed above that occurs during quality assurance tests. This damage is caused by penetration of fluids between the substrate and the encapsulating material or through the substrate. In addition, electrical connections on these boards are typically formed by solder bumps on the uncovered side of the circuit substrate. Such connections are difficult to use because they cannot be seen when the circuit board is viewed from the encapsulated side.